Generation and Observation of Tensile Waves in Oil Columns

Tensile stresses in liquids are not necessarily a familiar idea in fluid engineering, so much less accepted is the reality of tensile waves travelling through a liquid continuum. The present paper aims at giving evident proof for the premise. In a pipe with its end closed, branching from the main line where column separation is started, alternating tensile and compressive waves have been beautifully created and measured. It has turned out that the ordinary viscous wave equation can properly predict the time profiles of the masured pressures, which reveals that a tensile wave propagates in oil at the same speed as a compressive one. The observations by use of a transparent acrylic tube have also revealed the rupturing of an oil column under an excessive transient tensile stress; in the aftermath there has emerged a bubble which is usually mistaken for the cause of cavitation, but is actually brought about by diffusive separation of gas from the oil into the rupture.